Up cut or down cut or straight bits?

[Designer]

I understand the difference between these types of bits as described in the material that came with my BT32. I usually used straight bits on my router table. I am not sure what is best on the CNC though.

I like the fact that the down cut does not try to lift the work piece. So far, I do not see an issue with the lower edge using a down cut. What drives the decision to use an up cut bit? Also, when using an up cut how much concern is there with it lifting the piece.

I am cutting hard white maple and walnut that is usually .75" thick.

Thanks

[steve_g]

https://drive.google.com/file/d/0B8G...ew?usp=sharing
Here’s a link to an article I edited with the input of several Botters. It explains many basic things about router bits but is getting a bit dated now. Maybe it will answer several of your questions!
SG

[ntraub01]

Up until this past month, we nearly ALWAYS used down cut bits as we are always trying to limit the amount of tearout we get on our veneer plywood. I can safely say that 9/10 times we DO NOT use up cut buts because of the ragged top edge it produces. However, we are now starting to use compression bits and our assembly guys are loving them and we are resurfacing our spoil boards much less frequently. I think we just recently cut 25+ sheets of MDF and the spoil board looked like there was more missing than remaining and we were getting great cuts on both the top and bottom edges.

To answer your question about how much concern is there when using an up cut bit...well knowing that we use mostly down cut and compression bits, you VERY QUICKLY realize that you have to take some extra steps to secure your work piece. I'm operating on the principle that it is easier to OVER SECURE the piece than under secure it. The machine isn't going to assist you in keeping it down, it is going to want to pick that piece of and throw it across the room, speaking from recent experience!!

Overall, I love the down cut bits and I'm a firm believer in the compression bits and only use up cut bits when I absolutely have to.

Enjoy!

[Bob Eustace]

The most important use for upcut bits is FIRE PREVENTION. If you say drill a 1/4" hole using a 1/4" downcut there is nowhere for the chips to go so you get a ton of heat. If you ever accidentally use the wrong bit apart from smoke you will hear popping sounds. This is serious stuff and well covered in TJ's videos.

[Designer]

Thanks much. This helps a lot.

[Designer]

Steve G., I have found the article on router bits real useful. I am always figuring my chip loads. What I am finding surprising is how low the RPM's are and how high the feed rate is for some bits. I just picked up a Onsrud 60-246 3 flute 3/8 diameter down cut end mill. Onsrud lists the 60-200 series on their soft wood page. They don't list it or the 60-350 series on their hard wood page. Makes me a bit uneasy since I planned to use it on cherry, walnut, and maple. The chip load is .006-.008. I get 3.0 inches per second at 7500 RPM when I use the SB chip load calculator. Unless I am missing something that is 180 inches per minute. That seems real fast. Am I missing something or should that be okay? By the way, I usually do not cut deeper than .5 x bit diameter. From what I see the feed and speed is based on the bit cutting at 1D. When using my router table I never went to a depth equal to the bit diameter. Should I cut .375 deep using the CNC? Sorry, just being cautious and trying to learn.

[steve_g]

Phil…
I’m glad you found the article helpful…
Here’s my take on chipload… Chipload numbers are designed for industrial applications and the closer you can get to them the better , BUT, Shopbotters often don’t have the horse power required to get to the ideal numbers, or may not have the ability to hold the material tight enough for the forces that a proper chipload generates. Two things to watch, first, the temperature of the bit. It’s heat that destroys the carbide matrix material. You should be able to comfortably touch a bit that has just finished a toolpath. A hot bit needs to be fed faster or run at a lower RPM. Next, chip size. Watch chips themselves. Chips carry the heat of cutting with them while dust doesn’t. Get to recognize what a chip that leaves your bit cool looks like.
After you’ve been cutting awhile, you’ll know by the sound if the speeds are right! If you are uncomfortable with the speeds the chipload calculation comes up with, run some tests in scrap starting just below recommended chipload, progressing through to the high side. See what gives you the best results!
Just because a bit is listed for softwood doesn’t mean it won’t cut hardwood, BUT, if you’re cutting hardwood all day, get a bit whose angles have been optimized for the density of the wood you’re cutting.
Chipload and 3D and V carving… These are, in my opinion, almost meaningless numbers! Your machine spends so much of its time ramping into and out of corners that designated speeds are almost never reached! Since much of the time in this type of toolpath is the Z axis going up and down, I like to push this number a bit!
If I haven’t answered your questions, tell me so I can do a better job!
SG

[Designer]

Thanks Steve. I will run some tests. I was going to ask about the screaming bit issue, but when I read the article again I found it. I need more chip load. Thanks again. You have been a great help.

[scottp55]

Phil,
Just thought I'd mention that often in hardwoods just swapping to a "conventional" cut vs Climb will make a big difference.
Also "Downs" seem to prefer a much slower rpm than Straights or Ups in woods you mentioned.
Like Steve said, watch your bit temp and chips AND your cut quality.
Chipload calculator is only a guideline for ALL machines, pretty soon you'll be able to "Hear" what's right for yours.
Screaming means it hungry
You'll Get it!
scott